The present disclosure relates to a semiconductor device, for example, an associative memory.
A storage device called an associative memory or content addressable memory (CAM) is a device that searches for a data word matching a search word from data words stored in the storage device, and when finding a matching data word, outputs the address.
There are two types of CAM: binary CAM (BCAM) and ternary CAM (TCAM). Each memory cell of BCAM stores information of either “0” or “1”. However, in the case of TCAM, each memory cell can store information of “Don't Care” (an asterisk “*” is used in this example), in addition to information of “0” or “1”. Note that “*” indicates that either “0” or “1” is available.
The TCAM device is widely used for address search and access control in routers for networks such as the Internet. In order to deal with an increase in the capacity, the TCAM device is generally configured with a plurality of arrays to simultaneously perform a search operation on each of the arrays.
The TCAM device, which can simultaneously compare input search data (input packet) with TCAM cell data, is faster than the random access memory (RAM) in all search applications. However, a problem is that the power consumption is increased because a search current is generated in the search.
In this regard, Japanese Unexamined Patent Application Publication No. Hei 6(1994)-333395 (Patent Document 1) discloses a configuration that narrows down blocks to be search by inputting a block selection signal for selecting blocks in order to reduce the power consumption. However, it is necessary to provide the selection signal for selecting blocks in advance and the configuration is complicated.
Further, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2013-537680 (Patent Document 2) discloses a configuration in which no power is supplied to areas with no valid data based on the presence or absence of valid data (data word) in the memory array. However, when a data word is present, power supply is performed, so that it is difficult to obtain a sufficiently low power consumption effect.
Further, Japanese Unexamined Patent Application Publication No. 2003-272386 (Patent Document 3) discloses a TCAM device configured such that a plurality of sub-arrays are arranged in a match line direction and are coupled in a pipe line manner to reduce the power consumption. The TCAM device of this document performs a search, in the subsequent stage, only on entries that match in the previous stage.